UV EPROMs are commonly used semiconductor memory devices into which data may be written and which then retain the stored information indefinitely. Such memories may also be erased by exposure to ultraviolet light, rewritten and reused. A disadvantage of UV EPROMs is that they must be packaged in expensive packages with UV-transparent windows in order to allow the ultraviolet light to reach the memory cells.
Recently, a market has been created for OTP memory devices. OTP memories are UV EPROMs packaged in inexpensive plastic packages without windows. They may, of course, be programmed and used as any other UV EPROM, but may never be erased, rewritten and reused. For a variety of applications, OTP memories are cost competitive with mask-programmable memories, which OTP's resemble functionally. In addition to memory-only devices, microcomputers and other integrated circuit products which include OTP memory have become successful. As used hereinbelow, the term OTP memories will include both memory-only devices and devices in which memory is integrated with other circuits, such as a microcomputer.
Testing of OTP memories during the manufacturing process is particularly difficult. While the devices are still in the form of wafers or die, prior to being irreversibly sealed into plastic packages, they may be fully tested. That is, each memory cell may be programmed, read and erased to ensure its functionality. Since there is, as yet, no package surrounding the device, exposing it to ultraviolet light is no problem. However, once an OTP memory is packaged, it has not been possible to perform a test which guarantees that each cell is still capable of functioning. Since the packaging process involves one or more steps at relatively high temperatures, and perhaps other potentially damaging steps, there are always some memory cells which pass all tests prior to packaging, but which fail when a customer attempts to use the device.
One partial solution to the problem of post-packaging testing of OTP memories is to include "extra" memory cells in each device. These extra cells are tested after packaging and it is assumed that if the extra cells are good, the remainder are also good. Obviously, some bad memories will still pass final inspection if such a test method is relied upon.